This invention relates to the art of seismic data processing of data obtained from co-located pressure sensor and particle velocity sensors. More particularly, the present invention relates to removal of ghosting caused by water-column reverberation with such data.
For many years, "dual sensors" or co-located hydrophones and geophones have been used in seismic bottom-cable applications (as used herein, the term "hydrophone" is a synonym for a pressure sensor and "geophone" is a synonym for a particle velocity detector). Many have used the differing properties between the hydrophone and a geophone to effect wavefield separation, wherein the upgoing wavefield on the two sensors is equal while the downgoing wavefield is opposite in polarity. Thus, by adding the wavefields, the downgoing wavefield is canceled and the upgoing wavefield is retained. For example, see Ruehle, U.S. Pat. No. 4,486,865, issued Dec. 4, 1984 to William Ruehle, and incorporated herein by reference; U.S. Pat. No. 4,979,150, issued to Frederick Barr on Dec. 18, 1990, also incorporated herein by reference; U.S. Pat. No. 5,163,028, issued to Frederick Barr, et al. on Nov. 10, 1992, incorporated herein by reference; U.S. Pat. No. 5,235,554, issued to Frederick Barr, et al. on Aug. 10, 1993, and incorporated herein by reference; and U.S. Pat. No. 5,365,492, issued to William Dragoset on Nov. 15, 1994, also incorporated herein by reference. As recognized in the above-cited references, the problem of "water-column reverberation" continues to cause problems in the use of co-located hydrophone-geophone pairs. Each has problems and use relatively complex methods for eliminating water-column reverberation and the imperfect coupling between the geophone and the seabed floor.
None of the above-references, however, succeed in true matching of the hydrophone and geophone signals. One attempt of reverberation elimination failed to recognize the inherent similarities of the wavelet. Another solution involved firing a "calibration" shot. This method was imperfect because it was sensitive to the position of the wave source relative to the receiver. In order for the calibration to be effective, the wave source must be positioned directly above the receiver. Such a position is practically difficult to obtain. A later method presents a solution to the positioning problem which arises from "calibration" of the wave source, but then failed to consider changes in geologic and ambient characteristics of the subsurface reflections.
Accordingly, it is an object of this invention to combine hydrophone and geophone signals while eliminating the errors found in previous attempts at eliminating water-column reverberation. It is also an object of the invention to avoid the need for making a calibration shot after the cable has been laid. Further, it is an object of the invention to observe water-column reverberation without the need for experimental observation that is subject to the position of the calibration shot relative to the hydrophone/geophone pair. Further still, it is an object of the invention to eliminate water-column reverberation in a manner that is not susceptible to changing reflections and coupling characteristics of the sea-floor.